An optical information recording medium on which information can be only once recorded by means of a laser beam (i.e., an optical disc of write-once type) is known as a recordable compact disc (CD-R), and is widely used in practice. The optical disc of CD-R type generally has a multi-layered structure typically comprising a disc-shaped transparent substrate (support), a recording layer comprising an organic dye (often referred to as “recording dye layer”), a light-reflecting layer comprising a metal (hereinafter, often referred to as simply “reflecting layer”), and a protective layer made of resin over-laid in order.
Large recording capacity is often needed in recent days, but an optical disc of CD-R type does not always have fully satisfactory large capacity. Therefore, an optical disc having satisfactory large recording capacity has been desired and proposed. For example, a recordable DVD (i.e., recordable digital video disc, DVD-R), which information can be recorded on and read out from by means of a laser beam having a shorter wavelength than that for CD-R, was proposed. The optical disc of DVD-R type is formed by laminating two composites having a layered structure. Each of the layered composites comprises a transparent disc substrate provided with a guide groove (i.e., pregroove) for tracking of the laser beam, a recording layer comprising an organic dye, a light-reflecting layer, and a protective layer arranged in order. The layered composites are combined with an adhesive so that the recording layers would be placed inside. The pre-groove on the substrate of DVD-R has a width of 0.74 to 0.8 μm, which is less than a half of the width of the pregroove on a substrate of CD-R. Otherwise, the optical disc of DVD-R type is formed by combining a protective disc plate and the layered composites comprising a substrate, a recording layer, a light-reflecting layer and a protective layer arranged in order.
For writing (i.e., recording) and reading (i.e., reproducing) the information, a DVD-R is irradiated with a laser beam of visible wavelength region (generally having a wavelength of 600 to 700 nm). By the irradiation of the laser beam, irradiated area of the recording layer is locally heated to change its physical or chemical characteristics, and pits are formed in the irradiated area of the recording layer. Since the optical characteristics of the formed pits are different from those of the area having been not irradiated, the digital information is optically recorded. The recorded information can be read by reproducing procedure generally comprising the steps of irradiating the recording layer with the laser beam having the same wavelength as that employed in the recording procedure, and detecting the light-reflection difference between the pits and their periphery.
A conventional CD-R preferably has a relatively thick recording layer because it is thought to be effective in ensuring high sensitivity and good modulation with high reflectance.
Japanese Patent Provisional Publication No. 2-232832 proposes that the thickness of the recording dye layer of a recordable optical disc be set within ±10% of a thickness corresponding to an optical path which gives the maximum reflectance. The optical path of a recording dye layer giving the maximum reflectance is determined from a reflectance curve which is prepared using recordable optical discs composed of the same disc substrate, the same recording dye layer having varying thickness, and the same light-reflecting layer. In the publication, it is particularly recommended that the thickness of the recording dye layer be set to give an optical path within ±10% of a thickness corresponding to an optical path which gives the first maximum reflectance, which is detected in the reflectance curve counting from the data on the thinnest recording layer. In working examples, a CD-R has a recording dye layer of 130 nm thick, which gives the optical path corresponding to the first maximum reflectance.
The present inventors have studied on a recordable optical information recording disc of DVD-R type (which suits high-density recording more than a disc of CD-R) and found the following problem. It is known that it is effective in ensuring good modulation with high reflectance to make the recording layer relatively thick. However, the thick recording layer often gives an unsatisfactorily large jitter value. A large jitter value means that pits are not formed faithfully corresponding to the information signals, and accordingly that errors occur in the information reproducing procedure. Therefore, it is required to reduce the jitter value of a DVD-R for ensuring reliable high-density recording.
Accordingly, it is an object of the present invention to provide an optical information recording medium of DVD-R type (i.e., recordable digital video disc) having a relatively high reflectance and giving such a low jitter value that errors hardly occur.